1.2 Glossary
Apex: The specification may refer to the "apex of the eye" or "apex of the cornea." Referring to a cross section of an eye in FIG. 1, Apex 101 is the outermost point of the cornea. PA0 Reflecting PA0 surface: The specification may refer to a "reflecting surface." It is intended that the term indicate any surface of a unit under test that is suitable for reflecting light (visible or otherwise), such as a ball bearing, marble, or human cornea. PA0 Horizontal PA0 Meridian: The horizontal meridian is the profile of the cornea along a line containing the apex and is horizontal with respect to the imaging camera. PA0 Vertical PA0 Meridian: The vertical meridian is the profile of the cornea along a line containing the apex and is vertical with respect to the imaging camera. PA0 Z-axis: Generally, the Z-axis refers to an axis in parallel with the optical axis. PA0 Unit PA0 Under Test: The "unit under test" refers to a reflecting surface under examination by the topography system. A "unit under test" may be any reflecting surface such as an eye, calibration ball, ball bearing etc. PA0 Keratometer: An instrument for determining the curvature shape of the corneal surface which generally uses a placido or other illuminated target that is centered around the patient's line of sight. The reflection of a placido or other illuminated target by the patient's cornea or by the tear film on the anterior surface of the cornea is subsequently analyzed to determine the surface contour of the eye.
1.1 The History And Background Of Corneal Modeling
A number of forms of eye surgery involve a consideration of corneal surface topography. In radial keratotomy, for example, a number of cuts are made into the cornea in order to change its curvature and correct refractive power so that images focus closer to the retina.
While ophthalmic surgery is often successfully performed, the results obtained have been subject to variation occasioned by the particular operating "style" of the individual surgeon which dictates the number, location and depth of incision. Elements of subjective judgment are paramount. It would be useful to provide a device that could assist the surgeon in more quantitatively assessing pre-operative and post-operative corneal contours.
The present system relates to improvements in the art of photokeratometry and more particularly to the use of multiple camera view in combination with digital image processing techniques to ascertain the radius of curvature, refractive power, vertical profile of the cornea, and location of the apex.
An initial development in keratometry came from Gullstrand in 1896. Gullstrand disclosed the foundation for the current technology but his apparatus had no provision to compensate for aberrations in the optical system other than limiting the photographic coverage of the cornea to a 4 mm area. As a result, multiple exposures and calculations were necessary to map the corneal surface.
Much of the modern technique was developed by Amsler in 1930 and embodied in his "Photo-Keratoscope" which also required measurement and calculation as a separate step to derive the corneal shape data.
A standard instrument which is in common use for central optical zone shape measurement is the Bausch and Lomb Keratometer. Several companies offer similar devices with similar principles of operation. In these devices a single Mire image is projected on a small central portion of the anterior surface of the cornea usually 3 mm in diameter. The user is required to operate several controls to bring the optically split Mire images reflected from the cornea simultaneously into focus and alignment. In addition, the operator manually records the data obtained at two perpendicular axes. Other instruments are also available, such as the Haag-Streit Javal Schiotz device which measures only one axis at a time, but is slightly easier to use and tends to be more accurate in practice than the Bausch and Lomb system. In addition there exists a photographic system made by International Diagnostic Instrument Limited under the trademark "CORNEASCOPE" (and a similar system made by Nidek in Japan), as well as autokeratometers by several manufacturers. The CORNEASCOPE produces instant photographs of the reflection of a placido disc and requires a second instrument separate from the camera assembly to analyze the data. This system is fairly accurate, but expensive and tedious to use. The autokeratometers all are believed to be limited to a single zone of approximately 3 mm diameter and, in cases where the magnitude of the astigmatism is low, are inaccurate in their assessment of axes of astigmatism. Also available are three computer-direct systems which use conventional image analysis algorithms in conjunction with a mini-computer. These are the Corneal Modeling System (CMS) introduce in 1987 by Computed Anatomy, Inc. of New York, N.Y. and the ECT-100. introduced into the market by Visioptic of Houston, Tex. and a system using light emitting diodes disposed in concentric rings built by Zeiss of Germany. The placido disc-photo technique is regarded by some as superior to the Bausch and Lomb Keratometer because of the much greater amount of corneal surface analyzed form the placido reflection as opposed to the MIRES of the Keratometer.
A number of patents have been issued that relate to keratometers. U.S. Pat. No. 3,797,921 proposes the use of a camera to record the placido reflection from a patients eye. From this photograph, the radius of surface curvature of the cornea is determined at several points and calculated using a complex computer system. The use of a ground glass focusing screen with the small aperture of the optical system and large linear magnification makes use difficult and requires a darkened room for operation.
U.S. Pat. No. 4,440,477 proposes a method and device for measuring the corneal surface, comprising a slit lamp for illuminating the corneal surface, a camera for recording the reflection from the corneal surface, and a processor to calculate the image distance and the radius of curvature of the eye. The operation of the processor evidently is not detailed in U.S. Pat No. 4,440,477.
A more recent entry into the market is the "Corneal Modeling System" manufactured by Computed Anatomy Incorporated of New York which uses a light cone placido target in conjunction with a "frame grabber" to digitize and store for conventional image analysis the pictorial data. The placido is in cylindrical form and illuminated from one end. This cylindrical placido maintains a small aperture optical system creating a large depth of field of focus of the imaging system and, consequently, requires a sophisticated focus determining apparatus to assure accurate and reproducible image evaluation. This system is said to produce corneal thickness data using a scanning laser, as well as the surface contour, but is very expensive and does not readily lend itself to clinical applications which are increasingly cost driven.
The prior art systems generally rely on a front view of the cornea to provide all the expected data. In many respects, this limitation causes significant potential for error or impracticality. The current invention addresses many of the suggested problems by providing a side (temporal) view of the cornea in addition to the traditional front view.
1.3 Problems In The Prior Art
Most current placido based systems cannot provide a three space location of the apex because front-view-only systems operate using a 2-D virtual image. While using a 2-D virtual image, it may be impossible to determine the true Z-axis location of the apex and therefore a three space location of the apex may not be found. While certain laser aided systems have claimed some success in finding the apex, the process is believed to be simpler and faster using the placido based multi-camera system proposed by the current invention.
As with apex location, most current placido based systems technology apparently cannot provide true topography data because of the limitations of the front-view virtual image. However, like the apex situation, the invention addresses this problem by providing a side-view real image in which the true topography can found.
Another limitation in the prior art is the general inability to accurately profile the cornea. A front-view-only system suffers this limitation because there may be no way to accurately locate even a single point on the Z axis. The multi-camera system of the current invention addresses this limitation because a side-view camera is able to profile the Z axis.
In addition to the other limitations discussed, the front-view-only systems in the prior art may cause severe errors because geometrical phenomenon can result in an identical data collection for more than one eye shape. The current invention circumvents this problem by providing a second view to act as a checking mechanism.
Furthermore, the front-view-only systems of the prior art are difficult to use in that they inherently rely on the subjective skill of the operator for accurately positioning the equipment with respect to the eye. For even the most experienced operators, this manual positioning factor can cause serious repeatability problems. The current invention is not as susceptible to these problems because the ability to locate the apex allows a system design whereby the system may auto-calibrate and auto-position. Furthermore, prior art systems can be slow to calibrate and position. Therefore drying of the tear film may result and cause patient discomfort and distorted reflected data.
Lastly, the geometric constraints of single-view placido based systems can make it difficult or impossible to collect data as far out as the limbus. The current invention addresses this problem by using the side view to retrieve the data.